I don't think I've ever heard someone say that at the start of a phone call but it's common when communicating with two-way radios like:
"Alex for John, over"
"Go for John, over"
"<whatever Alex needs to say to John>"
I mostly heard that when working in large retail stores, usually the walkie talkies have an end-transmission sound que so we didn't actually have to say 'over' at the end